1. Field of the Invention
The subject invention relates to holographic displays and, more particularly, to apparatus for converting an off-axis illumination beam into a radially symmetric on-axis beam in a holographic display.
2. Description of Related Art
In the prior art, illumination of a hologram which must rotate has taken place from a point on or close to the axis of the hologram. Since the hologram viewing direction is on this same axis, the illumination source can block the view or shine into the face of the viewer. A compromise solution is to widen the angular acceptance angle of the hologram to accept illumination from a larger off-axis angle. Increasing the off-axis angle degrades the hologram by spreading the holographic image, which lowers the brightness and increases reflections from the environment. In addition, it increases the probability of spurious illumination of the hologram from other light sources. In the case of a rotating display such as a holographically produced rotating pointer, misalignment of the pointer can result from off-axis illumination. Also, the diffraction efficiency of the hologram, that is the brightness of the image, decreases as the hologram is rotated away fron the angle at which is was constructed.